Method of case hardening titanium and zirconium alloys

ABSTRACT

An article of titanium or a titanium-based alloy, or of zirconium or a zirconium-based alloy is case hardened by heat treatment for at least 12 hours at one or more temperatures in the range of 850° C. to 900° C. and at a pressure in the order of atmospheric pressure in an oxygen diffusion atmosphere. The oxygen diffusion atmosphere comprises a carrier gas such as argon which does not react chemically with the article the said temperature range and molecular oxygen. The concentration of oxygen as the oxygen diffusion atmosphere is in the range of 10 volumes per million to 400 volumes per million.

This invention relates to a thermal treatment method. In particular, itrelates to a method of case hardening an article of titanium orzirconium or of an alloy based on titanium or zirconium.

WO-A-96/23908 discloses a process for manufacturing a titanium articlewith a hardened surface for enhanced wear resistance comprising thesteps of exposing the article to an oxygen-containing environment;heating the article to a temperature that allows oxygen to diffuse intothe article; soaking the article at the temperature for a timesufficient to oxidise elemental metal at the surface and cooling thearticle to room temperature. The heating and soaking take place at about500° C., and the oxygen-containing environment is an atmosphere of air.

U.S. Pat. No. 5,316,594 relates to forming a hardened outer shell on arefractory workpiece using an argon-oxygen atmosphere containing from 1to 3 mole percent of oxygen. If the workpiece is of zirconium themaximum treatment temperature is 1400° F. (760° C.). If the workpiece isof titanium the maximum treatment temperature is 815° C.

U.S. Pat. No. 580,081 relates to the treatment of intermetalliccompounds of titanium and aluminium in an atmosphere containing 20% byvolume of oxygen.

U.S. Pat. No. 4,263,060 relates to the treatment of titanium articleswith oxygen at a sub-atmospheric pressure.

WO-A-99/04055 (The University of Birmingham) discusses the need toprovide engineering alloys of titanium or zirconium with a hard caseconsisting of a region of relatively high hardness maintained to acertain depth below the surface before dropping more steeply and thengradually to the hardness of the untreated core material. WO-A-99/04055discloses a method of case hardening an article formed of titanium,zirconium or an alloy of titanium and/or zirconium in which the articleis heat treated for a short period of time, typically from 0.3 to 0.6hour, in an oxidising atmosphere containing both oxygen and nitrogen(typically air) at a temperature in the range of 700 to 1000° C. so asto form an oxide layer on the article, and then further heat treatingthe article in a vacuum or in a neutral or an inert atmosphere at atemperature in the range of 700 to 1000° C. so as to cause oxygen fromthe oxide layer to diffuse into the article.

According to WO-A-99-04055 the case hardened article may then be surfacetreated by the method according to WO-A-98/02595 (The University ofBirmingham) so as to improve the tribological behaviour of the article.This surface treatment comprises gaseous oxidation of the article at atemperature in the range of 500 to 725° C. for 0.1 to 100 hours, thetemperature and time being selected such as to produce an adherentsurface component layer containing at least 50% by weight of oxides oftitanium having a rutile structure and a thickness of 0.2 to 2 μm on asolid solution-strengthened diffusion zone wherein the diffusing elementis oxygen and the diffusion zone has a depth of 5 to 50 μm.

The dual step oxidation/diffusion treatment of the method according toWO-A-99/04055 is difficult to control. A small variation in the amountof oxide formed in the first oxidation step can result in a significantdifference in the eventual hardness profile at the end of the diffusiontime in the vacuum or the neutral or inert atmosphere. The methodtherefore relies entirely on empirical control, thereby causingdifficulties if it is required to treat a range of articles of differentshapes and sizes.

According to the present invention there is provided a method of casehardening an article of titanium or a titanium-based alloy, or ofzirconium or a zirconium-based alloy, wherein the article is heattreated at one or more temperatures in the range of 850° C. to 900° C.and at a pressure in the order of atmospheric pressure in an oxygendiffusion atmosphere comprising (a) a carrier gas which does not reactchemically with the article in the said temperature range and (b)molecular oxygen, wherein the concentration of oxygen in the oxygendiffusion atmosphere is in the range of 10 volumes per million to 400volumes per million.

In the method according to the invention the rate of oxygen diffusionfrom the surface into the body of the article is a function of theoxygen potential, i.e. the partial pressure of oxidant in the oxygendiffusion atmosphere. The measurement in real time of the oxygen partialpressure of a heat treatment atmosphere is conventional in some heattreatments of ferrous workpieces and may be performed using commerciallyavailable instrumentation. Accordingly control of the oxygen potentialis a simple matter of appropriately selecting the mole fraction ofoxidant molecules in the oxygen diffusion atmosphere and, if necessary,adjusting the mole fraction in response to a real time oxygen potentialmeasurement.

The carrier gas is preferably a noble gas such as helium, xenon, neon orargon, or a mixture of one or more such noble gases. Argon isparticularly preferred. It should be noted that nitrogen reacts withtitanium and zirconium at temperatures in the heat treatment range andtherefore cannot be included in the carrier gas.

The method according to the invention is performed at a pressure that isapproximately the same as the prevailing atmospheric pressure, i.e. at apressure in the range of 1.0 to 1.2 bar.

Preferably the oxygen concentration is in the range of 75 to 300 volumesper million; more preferably the oxygen concentration is in the range of100 to 200 volumes per million. These oxygen concentrations arepreferred for the following reasons. At below about 75 parts by volumeper million, the rate of oxygen diffusion is undesirably low andtherefore the time required to complete the treatment is undesirablyhigh. At 500 parts by volume of oxygen and above there is too marked asurface oxidation which can inhibit diffusion of oxygen atoms into thearticle being treated and/or a spalled surface oxide is produced, acondition considered unacceptable for engineering components. Indeed, atoxygen concentrations of 5000 parts by volume per minute an impermeableoxide surface is rapidly formed. It is within the scope of theinvention, however, to increase the concentration and/or partialpressure of oxygen in the atmosphere at or near the end of the treatmentso as to form a visible surface oxide layer that improves thetribological properties of the article. Such formation of a surfaceoxide layer can be performed at the same temperature as the diffusion orat a lower temperature, i.e. at any temperature in the range 500 to 900°C. and employing an atmosphere having an oxygen concentration of atleast 5000 volumes per million.

The method according to the present invention is particularly useful incase hardening engineering components or other articles formed ofcommercially pure grades of titanium, of titanium-based alloys (α, α+β,or β alloys), of commercially pure grades of zirconium, and ofzirconium-based alloys.

When the article is required to have enhanced fatigue properties, it maybe subjected after heat treatment to a mechanical surface treatment,such as shot peening.

The method according to the present invention will now be furtherdescribed with reference to the following Examples and to theaccompanying drawings, in which:

FIG. 1 is a graph showing the Vickers hardness profile for samples oftitanium alloy treated at 850° C.;

FIG. 2 is a similar graph to FIG. 1 but showing the Vickers hardnessprofile for samples treated at 900° C.; and

FIG. 3 is a similar graph to FIG. 1 but showing the effect of treatmentat two different temperatures.

EXAMPLES

Materials—Ti-6Al-4V alloy was selected as the test material as thisalloy constitutes some 50-60% of the global titanium output. Samples ofGrade 5 Ti-6Al-4V (25 mm×50 mm×3.2 mm) were acquired with a 600 gritsurface finish. The chemical composition of the alloy is shown inTable 1. Prior to treatment each specimen was cleaned with 2% Alconox™aqueous detergent in an ultrasonic bath followed by an ethanol rinse andwarm air dry. The specimens were weighed to an accuracy of ±0.01 mgafter cleaning. TABLE 1 Chemical Composition of the Grade 5Ti—6Al—4VAlloy Element Al V C Fe N O H Y Ti at % 6.15 3.93 0.03 0.15 0.02 0.17 47<50 Bal. (by ppm ppm weight)

Test Apparatus—All of the thermal treatments were conducted in a highpurity alumina tube furnace at a temperature of either 850° C. or 900°C. During processing the atmosphere was maintained at a constant inletcomposition and flow of 3000 cc/min using a MKS 647B Multi-Channel GasController system. Two argon/oxygen mixtures were mixed to produce thecorrect atmosphere composition. The first mixture was “house” argon withless than 1 ppm oxygen. The second mixture was obtained from a certifiedpremixed cylinder containing argon with 1040 ppm oxygen. The temperaturewas maintained with an external thermocouple and monitored with aninternal thermocouple. Two samples were heat treated together and wereheld vertically in a specially manufactured holder to ensure uniformsurface exposure. At the outlet side of the tube furnace, an IllinoisInstruments oxygen analyser, Model 2550 was sued to monitor thecomposition of the flowing gas.

Procedure—After weighing, two samples were inserted into the centre oftube furnace in the specimen holder. Prior to heating the tube furnaceand samples were purged with house argon for 1 hour to obtain abackground level of less than 1 ppm residual oxygen in the system. Thisatmosphere was used during heating to 850° C. or 900° C. After thetarget temperature was attained, the inlet gas composition was changedto the test atmosphere. Samples were treated in atmospheres containingoxygen in the range 1 to 500 ppm for 24 hours at temperature. After the24 hour period the atmosphere was returned to 100% house argon andmaintained during furnace cooling to room temperature. In two cases, theatmosphere composition was further modified to a second oxygen levelduring the heat treatment period. These two tests were conducted for atotal of 28 hours, 20 hours at the first oxygen level and 8 hours at thesecond. As a baseline, samples were treated in argon containing lessthan 1 ppm oxygen for 24 hours. After the samples were removed from thefurnace, each was again weighed to an accuracy of ±0.01 mg.

Hardness and Microstructural Evaluation—The maximum surface hardness anddepth of penetration were measured using a Vickers hardness traverse at25 and 50 gram loads. The lower load was used primarily at the edge ofthe sample to eliminate the risk of cracking. Microstructural features,such as case depth, were observed by light microscopy after etching inKroll's etchant (2% hydrofluoric acid in water).

X-Ray Diffractometry—After the heat treatment was completed, the surfaceoxide layer of one of the samples from some select treatments wasevaluated. A Philips X'Pert PRO Multi-Purpose Diffractometer operatingat 40 kV and 50 mA was used. The 2-theta scan was from 20 to 100 degreesat a step size of 0.01 degree and a rate of 0.4 sec/step. The resultantdata was analysed using Philip's Analytical X'Pert Software to identifythe observed peaks.

Results

The hardness profile of specimens heat treated at 850° C. and 900° C. invarious oxygen concentrations are shown in FIGS. 1 and 2. The differencein the magnitude of hardening between the two heat treatmenttemperatures is obvious. The 900° C. treatments produced a much highersurface hardness and resulted in a greater depth of penetration for anequivalent exposure time, as would have been expected from Fick's Law.

The baseline treatment of 1 ppm of oxygen at 850° C. resulted in a veryminor hardness increase at the surface and the depth of penetration wasless than 75 microns. FIG. 3 shows the hardened alpha case for thisspecimen. In addition to the lack of significant hardening, only aslight surface scale was present. The average total weight gain for thetwo specimens treated at this condition was only 0.35 milligrams.

Increasing the partial pressure of oxygen from 1 ppm to 3, 10 and 25 ppmsignificantly changed the maximum surface hardness and depth ofpenetration. An incremental increase is clearly observed from 1 to 3 ppmand again from 3 to 10 ppm of oxygen in FIG. 1. At 25 ppm oxygen nosignificant change in the hardness profile was measured, but the depthof the alpha case is greater in comparison with the 10 ppm sample. Thetotal average eight gain for the three conditions of 3, 10 and 25 ppmwere 2.96 mg, 8.29 mg, and 18.44 mg respectively. The weight gaincorresponded directly to the degree of surface oxidation and the depthof penetration observed.

The highest oxygen concentration in argon used (500 ppm) resulted in aheavy oxidation layer in addition to the surface hardening. On removalfrom the furnace a good portion of the oxide scale spalled off of thespecimens. This significant oxide coating appears to have reduced theamount of oxygen penetration by acting as a diffusion barrier. In boththe hardness profile (FIG. 1), the depth of penetration is clearly lessthan that observed for both the 10 and 25 ppm conditions. Thiscombination of surface condition and hardness profile was not considereduseful.

None of the samples heat treated at 850° C. had either the maximum casedepth or the hardness profile required for maximum performance.

The heat treatments conducted at 900° C. produced an improvement in thedepth of penetration and the formation of the appropriate surface oxide.Five oxygen concentrations were evaluated at this temperature. Thehardness profiles obtained for oxygen concentrations of 25, 50, 100, 200and 500 ppm are shown in FIG. 2. It can be seen that, for theseconditions, the depth of penetration was in excess of 250 microns. Themaximum hardness obtained was in excess of 1000 Hv for some of theconditions.

It was found important not to let the temperature exceed 900° C.;otherwise undesirable internal oxidation occurred. The samples treatedat 900° C. with 100 and 200 ppm of oxygen clearly showed the depth ofhardening was twice that observed for the 850° C. heat treatments. Thesurface layer was essentially 100% alpha slowly changing to thealpha-beta microstructure as the depth increases.

The heat treatment at 500 ppm again produced a spalled surface oxide, acondition considered unacceptable for an engineering component. The 100and 200 ppm surfaces were fairly uniform and adherent and no spallingoccurred after removal from the furnace. The weight gains for the 900°C. treatments were significantly greater than those observed for the850° C. treatments. The average gains were 21.7 mg at 25 ppm; 58.1 mg at50 ppm; 68.4 mg at 100 ppm; and 85.0 mg at 200 ppm. Based solely on thesurface films produced and the depth of penetration this increase inweight was within the expected range.

Two double treatments were conducted to evaluate the effect of a moreoxidising condition (100 ppm oxygen) in combination with a lessoxidising condition (10 ppm oxygen). In a first treatment the sample wassubjected at 900° C. to 10 ppm oxygen for 20 hours, and then to 100 ppmoxygen for a further 8 hours. In a second treatment, the sample wassubjected to 100 ppm oxygen for 20 hours and then to 10 ppm oxygen for afurther 8 hours. Each of these treatments thus had a total exposure timeof 28 hours compared with the 24 hours for the single treatments. The 10ppm followed by 100 ppm condition produced a more uniform depth ofpenetration with a consistent hardness over the first 75 microns. Thesurface oxide was found to be adherent and uniform with a weight gainthat was measured to be 37.5 mg.

The second double treatment of 100 ppm followed by 10 ppm produced anextremely hard surface and an enhanced depth of penetration. It isbelieved that the lower partial pressure reduced some of the scaleformed during the initial 100 ppm exposure and allowed for furtheroxygen penetration. This treatment produced the greatest depth ofhardening. The average weight gain for these two samples was 62.1 mg, avalue slightly less than that observed for the single 24 hour 100 ppmtreatment (68.4 mg).

X-ray diffraction data revealed that some treatments did result inrutile, TiO₂, on the surface of the specimens. The single treatments at900° C. of 100 ppm and 200 ppm oxygen resulted in a rutile scale on topof the alpha case. The treatment at 850° C. and 10 ppm, which resultedin only a visible surface haze, did not exhibit any discernible surfaceoxides. However, examination of this sample showed that the alpha peakswere shifted due to the interstitial oxygen in the hexagonal closepacked lattice. This shift made identifying the alpha peaks in othersamples easier.

Two of the double treatment specimens were examined and found to havevery little rutile oxide on their surfaces. One specimen, 20 hours at 10ppm and 8 hours at 100 ppm, visually appeared to be the same as thesingle treatment 100 ppm specimen but only small amount of rutile wasdetected. Other oxides, Ti₂O₃ and Ti₉O₁₇ were more predominant. Thisresult indicated that 8 hours at 100 ppm oxygen is not enough to form auniform rutile scale. Another specimen, 20 hours at 100 ppm and 8 hoursat 10 ppm, did not exhibit any rutile scale at all. The scale was amixture of non-protective titanium oxides. Clearly the reduced 10 ppmoxygen level eliminated the surface rutile oxide that was present aftertreatment at 100 ppm. Visually this specimen appeared to have adifferent scale, a light grey colour compared to a dark blue-grey colouron the other samples. Although this treatment produced the greatestdepth of penetration, the surface scale was not ideal for engineeringapplications.

The single 900° C. treatments at oxygen concentrations of 100 ppm and200 ppm oxygen resulted in a rutile surface after 24 hours of exposure.The double treatment that ended with 8 hours at 100 ppm only formed asmall amount of rutile indicating that extended times are required toobtain the equilibrium oxide of rutile. Based upon the x-ray analyses,the two double treatments, although producing an excellent alpha-case,are not effective in forming duplex surfaces. It is believed that bymodifying the atmosphere to the oxygen composition of air (i.e. from 75to 85% by volume argon; 15 to 25% by volume oxygen) for the last 20minutes of treatment at 850° C. would produce a rutile layer of optimumthickness over the alpha case produced at 900° C.

1. A method of case hardening an article of titanium or a titanium-basedalloy, or an article of zirconium or a zirconium-based alloy, comprisingtreating the article with heat for a period of at least 12 hours atleast one temperature selected from the range of 850° C. to 900° C. andat a pressure in the order of atmospheric pressure in an oxygendiffusion atmosphere the atmosphere comprising: a) a carrier gas whichdoes not react chemically with the article in the temperature range, andb) molecular oxygen, wherein a concentration of oxygen in the oxygendiffusion atmosphere is in the range of 10 volumes per million to 400volumes per million.
 2. The method as claimed in according to claim 1,wherein the oxygen concentration is in the range of 75 to 300 volumesper million.
 3. The method as claimed in according to claim 1, whereinthe oxygen concentration is in the range of 100 to 200 volumes permillion.
 4. The method according to claim 1, further comprisingsubjecting the article to a further heat treatment at a temperature inthe range of 500° C. to 900° C. in an atmosphere having an oxygenconcentration of at least 5000 volumes per million so as to form avisible surface oxide layer on the article to improve tribologicalproperties of the article.
 5. The method according to claim 4, whereinthe atmosphere in which the tribological surface oxide layer is formedcontains from 15% to 25% by volume of oxygen and from 75% to 85% byvolume of argon.
 6. The method according to claim 1, wherein the carriergas is argon.
 7. A case hardened article of titanium, a titanium-basedalloy, of zirconium or a zirconium-based alloy provided by the methodaccording to claim 1.